1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device for mounting a device chip by a flip-chip bonding method and more particularly to a method of enhancing the purity of a solder ball thereby improving the manufacturing yield of the device.
2. Description of Related Art
For further proceeding size-reduction of electronic equipment, it is important how to improve the density of mounting parts. Also for semiconductor IC, wireless bonding of directly connecting LSI bare chips to conductor patterns on a mounting substrate has been proposed instead of existent package mounting using bonding wires and a lead frame. Among all, a method of forming all electrode portions, solder balls (bumps) and beam leads connected thereto on the device-forming surface of a device chip and directly connecting the same to a conductor pattern on a mounting substrate with the device-forming surface being downward is referred to as a flip-chip bonding method and utilized generally in the mounting of hybrid IC or in application uses of large-scaled computers since assembling steps can be rationalized.
In particular, solder balls are expected to take an important position more and more as mounting terminals for BGA (ball grid array) packages which are promising as multi-pin packages in the feature. BGA means a technique of converting an arrangement pattern of Al electrode pads usually concentrated in the periphery of a device chip into a regular arrangement pattern of electric contacts distributed in a wider range by way of an insulative intermediate layer (interposer) and disposing solder balls to the electric contacts. Since a large arrangement pitch can be ensured between adjacent solder balls by BGA, there is no worry of short-circuit between the solder balls and, accordingly, a device chip can be mounted on a mounting substrate with a sufficient bonding strength without reducing the ball diameter.
In recent years, solder balls are sometimes formed by as much as 200 or more per one package, and the reliability of the mounting depends on how such a multiplicity of solder balls can be formed at a uniform height.
The solder balls have generally been formed by electrolytic plating but the method involves a problem that the thickness of a solder film to be formed fluctuates depending on the surface state of an underlying material layer and slight scattering of electric resistance.
For solving such a problem, the present applicant has already proposed a method of forming solder balls comprising a combination of a vacuum thin film-forming technique and lift off of a resist pattern in Japanese Patent Laid-Open Hei 7-288255 previously. The method will be explained with reference to FIG. 7 to FIG. 10.
FIG. 7 shows a state of a wafer W in which a BLM film 15 is formed on an Al electrode pad 12 by way of passivation or a substrate 11, and a solder film pattern is formed by way of passivation by an organic protection film. ABLM (ball limiting metal) film is a sort of barrier metals formed with an aim of improving adhesion and prevention mutual of diffusion between the film and a solder film to be formed subsequently and this naming is derived from that the film determines the finished shape of a solder ball.
Referring simply to the steps so far, an Al electrode pad 12 is at first patterned to a predetermined shape on a substrate 11 in which all devices have been formed. Then, the entire surface of the wafer W is covered with an SiN passivation film 13, and the film was patterned to form an opening 13a facing the Al electrode pad 12. Subsequently, the entire surface of the wafer W is deposited with a polyimide film 14 as an organic passivation film, and an opening 14a facing the Al electrode pad 12 is formed further to the inside of the opening 13a.
Then, the BLM film 15 is formed so as to cover the opening 14a. The BLM film 15 is a multi-layered film comprising a Cr film, a Cu film, an Au film laminated by sputtering orderly from the lower layer, which is usually formed by a lift off method.
Then, a solder film pattern 17a is formed by a lift off method. At first, as shown in FIG. 8, a resist pattern 16 of a sufficient thickness having an opening 16a exposing the opening 14a and a region in the vicinity thereof is formed. Then, the entire surface of the wafer W is coated with a solder film. The solder film is formed at the inside of the opening 16a in contact with the BLM film 15 and separated into a solder film pattern 17a to form a solder ball in a subsequent step and an unnecessary solder film 17b deposited on the resist pattern 16 and to be removed in a subsequent step.
Successively, the substrate is immersed in a resist peeling solution and subjected to waving treatment under heating and, when the resist pattern 16 and the unnecessary solder film 17b are removed, a solder film pattern 17a remains as shown in FIG. 9.
Subsequently, a heat melting treatment referred to as wet back is applied. That is, after coating a flux on the surface of the solder film pattern 17a, when temperature is elevated stepwise in an N.sub.2 atmosphere, the solder film pattern 17a shrinks by its own surface tension to form a solder ball 17c in a self-alignment manner on the BLM film 15 as shown in FIG. 10. The final temperature to be reached in the wet back is about 340.degree. C.
Then, the wafer W is put to dicing, and when the solder ball forming surface of individual device chips divided from the wafer W are made downward and opposed to the mounting substrate, and the conductor pattern preliminarily soldered on the mounting substrate and the solder balls are aligned and heat melted, chip mounting is completed.
By the way, the thickness of the solder film pattern 17a that determines the size of the solder ball 17c is made sufficiently large with a view point of ensuring the bonding strength of the chip to the mounting substrate and the dimensional stability. The thickness is generally about 30 .mu.m although depending on the arrangement pattern of balls. Accordingly, the film thickness of the underlying resist pattern 16 for lift off is desirably greater than 30 .mu.m. The film thickness is several tens times as large as the thickness of the resist pattern used for the preparation of internal circuits of the device.
However, if the thickness of the resist pattern 16 is large as described above, a great amount of water derived from a liquid developer or cleaning water tends to be taken into the film after photolithography. Particularly, the water content taken into the deep inside of the film can not sufficiently be removed easily even by way of usual baking treatment. In addition, the resist pattern 16 may sometimes re-absorb moistures in air in the substrate cooling step after the baking treatment. If the solder film is to be vapor deposited in contact with the resist pattern 16 containing a great amount of residual water, a so-called degassing occurs in which water is gasified by the heating of the substrate upon vapor deposition and released from the resist pattern 16. The released water fluctuates the film-forming condition of the solder film and a portion of water is taken into the inside of the solder film pattern 17a as shown by arrows in the figure.
A portion of water taken into the solder film pattern 17a is also derived from a polyimide film as shown by arrows in the figure.
If wet back is applied to the solder film pattern 17a formed in such a situation, the water taken previously into the film may be gasified and expanded in the solder ball 10 to cause voids 18 or water may oxidize constituent elements of the solder film to locally deposit metal oxides 19. The voids 18 or metal oxides 19 increase the specific resistivity of the solder ball 17c after finishing or lower adhesion between the BLM film 15 and the solder ball to cause degradation of lower the manufacturing yield of the device.